If we're looking at cost effectiveness alone, the bike need not be expensive and most other investments, like accessories, should be transferable from bike to bike. So consider the bike as a whole a consumable component. Almost nothing on it need be required to last forever. Lights, racks, and choice nice components are all transferable. A decent rolling stock doesn't cost much. You can have a used bike off Craigslist anyday for less than $200 easy. A little TLC and you're on the move. Everything else either lasts a long time or is a one time investment. Special clothing isn't necessary. A change of clothes usually is, so be sure you can keep your cargo secure and dry. But all of that can be done with minimal expense without sacrificing quality and durability where it counts.